Heretofore, a number of patents and publications have disclosed means for controlling the intensity, contrast or dynamic range of a projection image, the relevant portions of which may be briefly summarized as follows:
U.S. Pat. No. 5,386,253 to Fielding, issued Jan. 31, 1995, and incorporated herein by reference in its entirety, discusses exemplary projection systems utilizing one or more spatial light modulators (SLMs).
U.S. Pat. No. 5,717,422 to Fergason, issued Feb. 10, 1998, discloses a display and method employing a passive light modulator, a source of light, and a control for controlling the intensity of light supplied to the light modulator to provide images of good contrast for both bright and dark scenes. A method of displaying an image, which uses a passive light modulating display apparatus, includes controlling the intensity of light illuminating the display apparatus as a function of a brightness characteristic of the image.
US-20040001184A1 by Gibbons et al., published Jan. 21, 2004 (and claiming priority from PCT/US01/21367 filed Jul. 2, 2001), teaches a system for addressing deficiencies of electronic, SLM-employing projectors. It does so using techniques described as being capable of providing images of sufficient overall quality that they may be used in venues instead of, or in addition to, traditional large-format film projectors without disturbing audience perception that the viewed images are of high quality. The publication describes techniques including pre-modulation, luminance compensation, and partial luminance compensation.
A data and/or video projector's light valve optical engine is generally a device that uses means of modulating a fixed or variable light source based on either the reflective or transmissive properties of certain imaging panels. These panels may be Liquid Crystal Display (LCD), Liquid Crystal on Silicon (LCOS & SXRD), a Digital Micromirror Device (DMD) or any other pixelized imager panel(s) system.
Most high-lumen output video projectors use an arc lamp for the illuminating source. In the case of LCD and LCOS versions of imagers the white light produced by such lamps is usually separated into the primary colors of red, green, and blue using dichroic color-separating optics. Arc lamp separated colors are then passed through a polarizing filter to work with polarizing beam splitters. The primary color beams are either passed through the LCD panels (a transmissive technology) or reflected from LCOS panels (a reflective technology). However imperfections in both the means of polarization of the beams and the inability of the imagers to completely block the illumination source results in a reduction of the contrast ratio (CR) of the image.
DMD imagers utilize non-polarized light from the illumination source, may or may not contain beam splitters and contain micro-mirrors that direct light through the lens or away from the lens as directed to form the image (another reflective technology). Primarily, diffraction & reflection of light from various planes within the DMD and less than perfect reflectivity of the mirrors themselves results in a reduction of the contrast ratio of the image.
For the purposes of this presentation optical components between the illumination source (lamp) and the outermost exit lens are considered the projector's “optical engine.” Optical engines include but are not limited to dichroic beam splitters (where applicable), polarizers (where applicable), imaging panel(s), re-combining optics (where applicable), light tunnels, light collimators, irises, etc.
The present invention is directed to a method and apparatus to increase the effective contrast ratio and brightness yields for all types of data/video digital light valve image projectors. This concept is partially based on the fact that the contrast ratio dynamic from such devices displaying a “bright” image is limited to a projectors simultaneous contrast capability (commonly measured as ANSI contrast ratio—hereinafter ANSI CR), and the fact that the ON/OFF contrast ratio limits a projector's “dark” image's dynamic to a point much less than a light-valve projector's ANSI CR capability. It is also based on the fact that current light valve projectors (LCD, LCOS, SXRD, DMD) have extremely limited ON/OFF contrast ratios, when compared to standard cathode ray tube (CRT) type projectors, and are in need of this design improvement.
CRT type projectors are able to maintain, for the most part, full ANSI CR regardless of the image's general level of illumination due to the variable intensity output capability of their tubes. However, light valve projectors have a steady state of illumination source (i.e. a lamp) that is modulated solely by the imaging device(s). As such it is necessary for the imaging devices, regardless of type (DMD/LCOS/SXRD, LCD, etc) within the projector, to generate all of the image's dynamics. Since all these imaging devices “leak” light to varying degrees (i.e., areas intended to be dark or off are not completely dark), this limits the projector's ability to maintain full ANSI CR, particularly at the lower intensity levels resulting in a lack of depth in the image.
Attempts have been made in the past to vary a projector lamp's output to boost on/off CR, but these have failed to provide significant improvement due to the limited variable light output range of lamps (maintaining sustained ignition) and the fact that varying the lamp intensity drastically changes the color balance (balance in spectral output) of the lamp, thus limiting most light-valve projectors to one or two illumination levels from their bulbs (current examples: bright and economy-lamp modes). None of the lamp intensity schemes interact in concert with the imager(s) or help to produce better engine contrast ratio yield as the invention described herein does, neither do projectors with simple fixed or manually or electrically adjustable irises. The present invention relies on the technological premise that digital imaging devices perform three basic functions, among others, in order to generate a usable image for display:                1) modulate imager(s) with a source signal to create a recognizable pattern (i.e. an image);        2) set the general level of scene illumination; and        3) “paint” the image to create color(s) with a variety of available techniques.        
The present invention focuses on the second function set forth above—setting the general level of scene illumination, where this function is removed, at least partially, from the duty list of the imager(s) for the primary purpose of increasing overall contrast ratio and increasing the available ANSI dynamic for low-brightness images/scenes. In order to accomplish this function outside of the typical projection optical imaging engine, one aspect of the invention is intended for implementation in two stages, which are described in more detail below.
A first component of the present invention is a variable luminance control mechanism (VLCM). The VLCM is, in one embodiment, a special high-speed, temperature-resistant, electronically controllable iris system placed before, after or inside the optical engine of any light-valve projection device. A single (or multiple in some cases) iris system will be located at the point(s) either pre- and/or post-imager(s) within the optical engine that yields the best balance of results. This location will vary from projector to projector depending upon its particular design and on the intended results. In the case of single digital light processing (DLP) chip optical engines, this calls for a single iris system placed post-imager at a focal convergence point located post imager. The purpose of this adjustable iris is to vary the general scene illumination level, as the input signal varies, at a speed that is generally undetectable to the human eye. The main benefit of the luminance control function provided by the iris is extending the ON/OFF contrast ratio well beyond the ability of the imaging devices themselves. In other words, the use of the iris improves the ON/OFF contrast ratio by lowering the scene illumination on dark scenes to a nearly completely off level and thereby reducing the light “leaking” through the optical path of the projector. As will be described below, the use of one, or multiple irises, may also be employed to modulate or compensate for lamp brightness, including changes in or decay of the lamp/illumination source.
The addition of an iris does not change the color balance of the illumination (lamp) source or the imager(s); it is spectrally neutral in action. The shape of the iris may also be changed to assist in contrast ratio yields. For example, an oval or “cats eye” shaped iris may lend itself to a better contrast ratio yield than round or multi-sided (polygon) versions. Moreover, the present application contemplates that future implementations could also use extremely fast reacting photosensitive optics that would variably turn darker or lighter to either enhance or replace the mechanical iris method.
A second component employed in the present invention is an Adaptive Luminance Control Module (ALCM), which is coupled to the VLCM. The ALCM is a video signal processing system including circuitry and components that will operate and set the variable aperture opening or opacity of the VLCM and provide a corrected video signal to the input of the projector. This electronic luminance processing will follow the video input signal, tracking either the general (average) illumination level or the brightest point(s) in the signal (i.e., peak level detection) or any combination, and will output two different types of signals:                1) The VLCM drive signal. When fed a resultant, processed (analog or digital) signal, the VLCM will set the general scene illumination level. This optimizes both the engine's contrast (both absolute ON/OFF and ANSI CR) and the lumen output for the particular level of illumination that is needed to accurately reproduce the image. The VLCM drive signal is, effectively proportional to the input intensity of the video image (average or peak). The present invention contemplates the possibility that VLCM feedback may be required, and may employ one or more sensing mechanisms or circuits to indicate VLCM position or condition.        2) The image signal. The video output signal, post processing, is passed on to the projector's imager stage input. This image signal is processed and manipulated to take full advantage of the VLCM optical restriction capability. This unique relationship is described below. The processing for the image signal is primarily look-up (gamma) and gain based, along with a “black level clamp”. Output gamma will not track identically to the input signal's gamma; in other words, it will “adapt” to the input signal's illumination dynamics for optimization with the iris. There are various algorithms that will enable the desired functionality and will improve the technology. As used herein, “black level clamp” is a term describing the input to output signal proportion at 0 IRE. In other words, no matter what function the gamma tables and algorithms perform on the video signal; “0” input always equals “0” output.        
In accordance with the present invention, there is provided an apparatus for improving the operation of a digital image projector, comprising: a video input board of the projector; a optical engine of the projector, said optical engine receiving video signals and generating a light output for at least one primary color from; optics for transforming the light output from said light engine to a focused image for projection to a display screen; a variable luminance control mechanism (VLCM), associated with the optics, for receiving the light output and provide a correction thereto; and an adaptive luminance control module (ALCM) or processor, for receiving signals from said video input board, said adaptive luminance control module producing a signal on a VLCM bus connecting the variable luminance control mechanism and the adaptive luminance control module, said signal causing the variable luminance control mechanism to change the luminance of the light output and provide a corrected video signal from the projector.
In accordance with another aspect of the present invention, there is provided a method for improving the operation of a digital image projector, comprising the steps of: receiving image signals from a video input board in the projector; using an adaptive luminance control module, producing an output signal on a VLCM bus connected to the adaptive luminance control module, said signal providing control information for a variable luminance control mechanism, located within the optical path of the projector; and adjusting the variable luminance control mechanism to produce a corrected light output from the projector.
In accordance with yet another aspect of the present invention, there is provided an apparatus for improving the operation of a digital image projector, comprising: a video input board of the projector; a light engine of the projector, said light engine receiving video signals and generating a light output for at least one primary color there from; optics for transforming the light output from said light engine to a focused image for projection to a display screen; a variable luminance control mechanism, associated with the optics, for receiving the light output and provide a correction thereto; and an adaptive luminance control module, for receiving signals from said video input board, said adaptive luminance control module producing a control signal, wherein said variable luminance control mechanism operates in response to the control signal to change the luminance of the light output and provide a corrected video signal for the projector
In accordance with another aspect of the present invention, there is provided a method for improving the operation of a digital image projector, comprising the steps of: receiving video signals from a video input board in the projector; and producing a first signal, using an adaptive luminance control module, to provide control information for a variable luminance control mechanism located within the optical path of the projector and a second signal which is a modified video signal, whereby the variable luminance control mechanism operates to produce a modified light output from said digital image projector.
One aspect of the invention is based on the discovery that the general level of scene illumination may be adjusted in a video projector to improve the effective contrast ratio. This discovery avoids problems that arise in conventional light projectors due to light leakage, etc. Using aspects of the present invention, overall contrast ratio and the available ANSI dynamic for low-brightness images/scenes may be significantly increased. In order to accomplish this function outside of the typical projection-imaging engine, this aspect is implemented using the ALCP and VLCM described herein.
The techniques described herein are advantageous because they can be adapted to any of a number of light projectors. As a result of the invention, it is possible to produce digital light projection systems with improved overall contrast ratios and available ANSI dynamics for low brightness images/scenes.
The present invention will be described in connection with a preferred embodiment, however, it will be understood that there is no intent to limit the invention to the embodiment described. On the contrary, the intent is to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.